Natural gas vehicles (NGVs) operate on the same basic principles as other internal combustion-powered vehicles. Fuel, in the form of natural gas, is mixed with air and fed into a cylinder where the mixture is ignited to move a piston up and down. Natural gas can power vehicles currently powered by gasoline and diesel fuels. However, at standard temperature and pressure, natural gas is a gas rather than a liquid. This gives rise to two types of NGVs, namely: those that are configured to use compressed natural gas (CNG); and those that are configured to operate on liquid natural gas (LNG).
In applications where weight and vehicle range are a concern, LNG is often the preferred fuel over CNG. Systems designed for storage and use of LNG operate at much lower pressures and can typically store as much as 2.5 times the fuel in the same space as conventional CNG systems.
However, to maintain and use LNG in a liquid state, the fuel must be transported and stored at cryogenic temperatures, typically with vacuum-insulated storage tanks. Refilling the cryogenic tanks is typically accomplished by transferring LNG from a transport vehicle and typically requires a significant amount of venting of natural gas in the process. Release of natural gas into the atmosphere is undesirable for a number of reasons. For instance, natural gas is highly flammable, and thus its release is a safety hazard. Natural gas is also a greenhouse gas. Finally, natural gas that is released is lost fuel, and thus lost revenue, for the LNG supplier.